CDMA cellular and PCS telephone networks are a type of mobile communication network that use psuedo-noise digital codes (“pn-codes”). Pn-codes spread the transmitting signals' spectrum so that multiple communications can occur over the same channel. This is achieved by multiplexing and demultiplexing a particular communication signal within the common channel. This process allows many communications to occur simultaneously. A plurality of base stations use the same pn-codes with each base station assigned a different initial phase of code sequences.
The synchronization of multiple base stations is achieved by the use of the Global Positioning System (GPS) receivers at each base station location. Aided by appropriate stable clock generators, these receivers supply accurate timing information to the base station. Thus, it is important to measure the phase offset of each base station during initial installation and routine maintenance in order to ensure the integrity of the network. Testing devices, for example, pn-scanning receivers, used for this purpose must have a means to synchronize their timing to the absolute time used. An accurate, universally available timing information source, for example the GPS, is required.
These testing devices also assess the adequacy of signal coverage within a service area for cellular and PCS network as well as measure the quality of the CDMA signals from a base station. One measurement device is the pilot scan. The pilot test scanner requires access to an external, accurate clock. The Global Positioning System, GPS, is used as the clock source in the typical pn scan device. As such, these test devices have GPS receivers.
One of the methods used for the synchronization of the test receivers for CDMA pn-scanning requires a GPS receiver as part of the test instrument. Wherever the GPS signals are available, that is in most outdoor settings, this method provides an accurate and convenient way of synchronization. However, it is not practical for settings where there is no reliable GPS coverage, such as indoor settings, some urban locations and wherever access to GPS is obstructed.
Another possible approach relies on the information contained in the CDMA signal itself to derive the accurate frequency and timing information. Berkeley Varitronics Systems sells The Hummingbird™ a handheld PN scanner suited for indoor microcell IS-95 analysis. The Hummingbird demodulates the synchronization channel and reports base station ID, time and date all without use of the GPS. However, this approach suffers from an inherent flaw, since it relies on the accuracy of the base station's signal for the testing of the same signal. Additional problems associated with this approach are the uncertainty caused by an unknown propagation delay from the base station and the complexity of the implementation.
U.S. Pat. No. 6,101,176 was issued to Honkasalo et al. for a “Method and apparatus for Operating an Indoor CDMA Telecommunication system” addresses the issue of indoor and outdoor CDMA competing with each other. Honkasalo proposes that the indoor system monitor the outdoor system to identify those radio resources not in use or interference free so that the indoor system can utilize those resources.
U.S. Pat. No. 6,058,136 was issued to Ganesh et al. for a “System and method for PN offset index planning in a digital CDMA cellular network.” This invention describes a system and method for establishing PN offsets so as to minimize interference. One of the parameters chosen is designed to optimize the pn scan rate.
U.S. Pat. No. 5,945,948 issued to Buford et al. for “Method and apparatus for location finding in a communication system” is concerned about being able to locate a subscriber by measuring the user's cell phone transmission signal's angle of arrival and time arrival at one or more CDMA base stations. From this information the user's location can be approximated. Since a more accurate determination can be made with multiple measurements of the subscriber's transmission signal, one embodiment of this patent is to augment base stations with PN scanners so as to increase the coverage for purposes of locating subscribers. These scanners would be equipped with GPS receivers so as to synchronize the scanners' clocks. Part of the analysis requires calculating the time it took the signal to traverse from transmitter to receiver.
U.S. Pat. No. 5,398,276 was issued to Lemke et al. for “Cellular-system signal-strength analyzer”. Lemke was concerned about measuring signal strength of radio frequency cellular telephone transmissions in locations frequented by pedestrians including indoors locations and charting the signal strength at different geographical positions. Geographic positioning is calculated using a laser range finder along with a portable computer. Standard AMPS scanner, carried in a backpack is used to measure RF signal strength. AMPS (Advanced Mobile Phone Service) is concerned with analog, frequency division multiple access cellular systems and as such would not be concerned with time synchronization.
U.S. Pat. No. 5,590,177 issued to Vilmur et al. for “Method for preventing a dropped call during a handoff in a radiotelephone system” addresses the problem of dropped calls when one base station hands off a cellular call to a new CDMA base station. The new CDMA base station signal strength to power ratio is measured at the receiver location. As the ratio diminishes to some threshold level, a signal would be sent to the original CDMA cell location indicating that an alternate base station is required to maintain the call. A PN scanner is used to measure the signal to power ratio at the receive location.
Testing CDMA signal propagation and coverage using a PN scanner is well known. However, when the test location does not have ready access to a stable, accurate timing synchronization source, the test results may be suspect.
What is required is a method and apparatus to provide a simple method and apparatus for testing CDMA signal propagation and coverage, including pn offset measurements, in indoor and similar settings with no or limited access to GPS signals.